The present invention relates to a smoothing circuit used for a switching power supply for charging a battery and so on, and formed of a combination of a reactor and a capacitor.
In a smoothing circuit for a switching power supply formed of a combination of a reactor and a capacitor, a high switching frequency is used, and a film capacitor or a ceramic capacitor is used instead of an aluminum electrolytic capacitor in order to increase its life expectancy and reduce its size. FIG. 8 is a connection diagram showing an embodiment of a DC-DC converter section in a conventional switching power supply. In FIG. 8, 1 is a transformer; 2 is a switching element; 3 and 4 are diodes; 5 is a reactor; 6 is a capacitor; and 7 is a resistor. A control circuit for driving the switching element 2 is omitted. When a film capacitor or a ceramic capacitor is used as the capacitor 6, since its internal equivalent resistance is small, a control system for the DC-DC convertor section may become unstable. To stabilize the control system, the resistors 7, which have a small resistance value due to its parallel connection, are connected in series to the capacitor 6.
In addition, if a high-performance aluminum electrolytic capacitor is used as the capacitor 6, when the ambient temperature decreases to approximately -55.degree. C., the internal equivalent resistance increases three to four times as compared to that at a room temperature (20.degree. C.). Thus, to reduce the ripple voltage in the output at a low temperature or to reduce the voltage decrease caused by a rapid change in load, a film capacitor or a ceramic capacitor is connected parallel to the aluminum electrolytic capacitor.
When a battery is connected to an output end of a switching power supply for charging, in view of the situation that it is mistakenly connected in a reverse polarity or a battery having a voltage higher than the output voltage is mistakenly connected, the aluminum electrolytic capacitor, which is likely to be affected by a reverse voltage or an overvoltage, must be protected. FIG. 9 is a connection diagram showing another embodiment of a DC-DC convertor section of a conventional switching power supply. In FIG. 9, 12 is a diode; 13 is a relay coil; 13A is a contact of the relay coil 13; 21 is a battery of a normal polarity; and 22 is a battery of a reversed polarity. The other components are the same as those shown in FIG. 8 and have the same reference numerals. The relay coil 13 is excited to close the contact 13A only when the battery 21 is normally connected. If the battery 22 is connected in a reverse polarity, the diode 12 blocks an exciting current, so that the relay contact 13A remains open, thereby precluding a reversed voltage from being applied to the capacitor 6.
FIG. 10 is a connection diagram showing yet another embodiment of a DC-DC convertor section of a conventional switching power supply. In FIG. 10, 11 is a relay coil; and 11B is a contact of the relay coil 11. The other components are the same as those shown in FIG. 9 and have the same reference numerals. As an example of an overvoltage application, this figure shows two batteries 21 mistakenly connected in series. If one battery is used, the relay 11 does not operate, whereas if two batteries are used, the relay 11 operates to open the relay contact 11B, thereby preventing an overvoltage from being applied to the capacitor.
In a smoothing circuit for a switching power supply formed of a combination of a reactor and a capacitor, if a film capacitor or a ceramic capacitor is used to increase the life expectancy of the circuit and reduce its size, a resistor having a very small resistance value must be connected in series to such a capacitor. If the resistance value is small, that is, approximately 5 to 10 m.OMEGA., 10 to 20 commercially available 100 m.OMEGA. resistors must be connected in parallel, thereby requiring a relatively large area on a printed circuit board to mount the parts thereon and increasing the parts and processing cost for the resistors and printed circuit board. In addition, in a shunt resistor, such as a manganic wire, since the resistance value depends on the length of the resistor, a certain wire length is always required to thereby increase the inductance of the resistor, thus increasing the output ripple voltage of the switching power supply. Accordingly, it is a first object of this invention to provide a smoothing circuit with an inexpensive resistor that is connected in series to a capacitor having a small equivalent internal resistance and that has a very small resistance value.
In addition, when an aluminum electrolytic capacitor is used, if the temperature increases, the equivalent internal resistance of the capacitor decreases to cause the control system unstable, thereby causing the output voltage to oscillate or increasing the output ripple voltage. In addition, when a high-performance aluminum electrolytic capacitor is used, in case a film capacitor or a ceramic capacitor is connected parallel to the aluminum electrolytic capacitor to reduce the ripple voltage in the output at a low temperature or to reduce the voltage decrease caused by a rapid change in load, if the electrostatic capacity of the film or ceramic capacitor is increased, the combined equivalent internal resistance of the capacitors is reduced. Accordingly, the control system becomes unstable at a high temperature, thereby causing the output voltage to oscillate or increasing the output ripple voltage. Thus, it is a second object of this invention to provide a smoothing circuit with a resistor that allows the control system to operate stably even at a high temperature if a capacitor, the equivalent internal resistance of which varies with the temperature, is used.
In addition, in a capacitor protection method using a relay, such as that shown in FIG. 9, when a battery of a normal polarity is connected and the relay is then turned on, a high charging current flows from the battery to the capacitor, thereby reducing the life expectancy of the relay contact or fusing the relay contact. Thus, it is a third object of this invention to provide a smoothing circuit, wherein the life expectancy of a relay contact used for a reverse-voltage protection circuit is increased.
In addition, in a capacitor protection method using a relay as shown in FIG. 10, the operational delay of the relay causes an overvoltage to be applied to the aluminum electrolytic capacitor for a short period of time, thereby causing the capacitor to be damaged or destroyed. Thus, it is a fourth object of this invention to provide a smoothing circuit with an overvoltage protection device that is not subjected to a time delay.
Also, in an overvoltage protection method using a relay as shown in FIG. 9, if the switching power supply stops operation in a condition that the battery is connected in a normal polarity, a relay exciting current continues to flow from the battery to cause the battery to consume unnecessary power. As a result, the electromotive force of the battery may be lost if it takes a long time to perform the stopping operation of the switching power supply. Thus, it is a fifth object of this invention to provide a smoothing circuit with a reverse-voltage protection circuit that can prevent the power of the battery from being consumed while the switching power supply is not in operation.